LanguageListBuilder::submitForm

public LanguageListBuilder::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  parent::submitForm($form, $form_state);

  // Save the default language if changed.
  $new_id = $form_state->getValue('site_default_language');
  if ($new_id != $this->languageManager->getDefaultLanguage()->getId()) {
    $this->configFactory->getEditable('system.site')->set('default_langcode', $new_id)->save();
    $this->languageManager->reset();
  }

  if ($this->languageManager instanceof ConfigurableLanguageManagerInterface) {
    $this->languageManager->updateLockedLanguageWeights();
  }

  drupal_set_message(t('Configuration saved.'));
  // Force the redirection to the page with the language we have just
  // selected as default.
  $form_state->setRedirectUrl($this->entities[$new_id]->urlInfo('collection', array('language' => $this->entities[$new_id])));
}